1. Field of the Invention
The present invention relates to an apparatus and method for receiving TV signals, especially to a TV receiver and an analog signal processing method for implementing intermediate frequency filtering of an analog TV signal in the digital domain.
2. Description of the Prior Art
A SAW filter is a frequency-selective device that is made by utilizing surface acoustic wave (SAW) effect and harmonic oscillation. It is well known in the industry that a SAW filter is a frequency-selective device, which passes signals of a certain range of frequencies and filters out signals of other frequencies. A SAW filter has advantages such as small size (only 1/40 of a ceramic dielectric filter), light-weighted (only 1/30 of a ceramic dielectric filter), and good frequency selectivity. Due to these advantages, the SAW filter is broadly utilized in certain fields such as those of communication systems, broadcasting, and TV.
Generally, a traditional TV receives analog channels, but recently a digital channel TV system that broadcasts better-quality digital TV programs has been introduced. When dealing with analog TV channels and digital TV channels, analog TV signals and digital TV signals are processed separately and respectively because analog and digital signals are essentially different in characteristics. Before digital TVs can completely substitute analog TVs, the present TV products must be able to receive and process analog TV signals transmitted through analog channels as well as digital TV signals transmitted through digital channels. Therefore, a receiver contained in a TV must have two separate demodulators, one for digital TV signals and the other for traditional analog TV signals.
Please refer to FIG. 1, which shows a typical TV receiver 100. The TV receiver 100 receives both analog and digital TV signals. After processed by a tuner 110, a radio frequency signal RF is transformed into an intermediate frequency (IF) signal IF, which is further processed by a SAW filter to suppress adjacent channel interference and perform channel selection. However, because of the significant difference between the characteristics of an analog TV channel and a digital TV channel, the TV receiver 100 generally contains two filters: an analog TV intermediate frequency SAW filter 122 and a digital TV intermediate frequency SAW filter 124, for respective SAW filtering operations. Please refer to FIG. 2, which shows a frequency response of the analog TV intermediate frequency SAW filter 122 and the digital TV intermediate frequency SAW filter 124 of FIG. 1. The solid line represents the frequency response of the digital TV intermediate frequency SAW filter 124, and the dotted line represents the frequency response of the analog TV intermediate frequency SAW filter 122. Obviously, these two frequency responses are not quite the same. At frequency f1 the analog TV intermediate frequency SAW filter 122 shows an audio carrier suppression, and between frequencies f2 and f3 the frequency response of the analog TV intermediate frequency SAW filter 122 corresponds to a Nyquist slope.
In the TV receiver 100, analog TV channels and digital TV channels must be separated into two paths for further video and audio processes. The intermediate frequency IF is transmitted to the analog TV intermediate frequency SAW filter 122 to generate an analog TV input signal Sa and to the digital TV intermediate frequency SAW filter 124 to generate a digital TV input signal Sd. However, because SAW filters are rather expensive, the TV receiver 100 becomes extremely costly.